The National Guard is part time so you can go to school and get excellent education benefits to help pay for your college.
The Marines - unless you go Reserves is full time. If you go Reserves you won't get the same great educational benefits that you would get from The National Guard.

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There is no "Better" here. It's a matter of preference. However, you may find that finding an aviation mechanic slot is much more difficult in the Marine Corps than it is in the Army National Guard. We certainly have more avaliblility in that arena over the Marines. Why not ask a local recruiter to see how far away a Guard aviation unit is located, and see if you can visit the Aviaition Support Facility near you for a tour.